Rewrites the README as an introduction

[johnnyt]

Rewrites the README as an introduction to the package: what it is, why it exists, how to install it, one basic usage block, a map of the documentation grouped by the reader's question, and what it is compatible with. No file under lib/ or src/ changes.

What changed

• What (under the H1): three sentences - a small, safe predicate language for Elixir; rules compile to instructions a stack machine runs; short programs run the same way.
• Why a predicate language of its own: one paragraph on the problem before (every condition in code, or text evaluated as code) and the difference after (the rule is data, parsed by a closed grammar, stored as plain terms, evaluated with no eval). No history, no predecessor, no date.
• Installation: the pin stays {:predicator, "~> 9.4"}.
• Basic usage: one code block in the library loan - a renewal rule compiled once, evaluated against two loans, a missing value read back as an error value, and a short program recording the renewal. It is a doctest: test/docs_examples_test.exs still reads README.md.
• Documentation: links grouped under Learn, Do, Look up and Understand, one line per link, no empty sub-list. The decision records folder is linked from Understand, beside the two records the old README cited.
• Compatibility: Elixir 1.18 or later, no runtime dependencies, and how a stored rule's instruction-set version relates to the package version.
• Contributing keeps the old Development section's pointer to the contributing guide.

The README goes from 337 lines to 112 (wc -l, base and head).

Unchanged except

Every H2 the old README had, by grep '^## ' base to head, and where its text now lives:

• Installation, Documentation, License: kept (Documentation rewritten as the grouped map).
• Quick Start: replaced by Basic usage, re-set in the library loan.
• A worked example: an authorization rule: its compile-once, error-value and positions material is in the basic usage block and in the existing embedding guide (choose a compile function, store the instruction list, runtime errors and positions). Its short-program subsection (execute/3, execute_value/3, :protected_roots) moves to the language reference as "Running a program from Elixir", re-set in the library loan; that page is already doctested by test/docs_examples_test.exs, so the snippets stay executed.
• Embedding Predicator in a host application: the provider, host slot, put_host/2 and serializable-context material already lives in the custom functions guide and the embedding guide, both linked under Do. Its editor-completion subsection moves to the language reference as "The vocabulary for an editor", with its snippets, under the same doctest.
• Migrating from =: covered by the language reference's = note and the architecture page's grammar-break section; the record is linked under Understand.
• Cross-Language Siblings: covered by the architecture page's section of the same name (linked under Understand) and the porting guide (linked under Do).
• Development: becomes Contributing.

The AST reference is linked by absolute GitHub URL because the package tarball does not ship it (the doc links stage refuses a relative README link to an unpackaged file); every other page link is relative, as before.

ex_quality and the README stage

The acceptance turns the README stage on only where ex_quality 0.16.0 is on Hex. It is not (Hex's latest is 0.15.1), so this PR takes the acceptance's fallback: the ex_quality dev dependency and .quality.exs are untouched, and turning the stage on at severity: :error is left as follow-up work.

Checks

readme-lint.rb README.md --manifest .claude/diataxis.md:

README.md: 112 lines, every part present

readme-lint.rb README.md (no manifest):

README.md: 112 lines, every part present

• Full mix quality green on the committed tree: Format, Compile, Doc links, Dependencies, Docs, Credo, Dialyzer and Tests (2,914 of 2,914, 95.5% coverage); Doctor, Gettext and Sobelow skipped as not installed, the repo's expected state.
• test/docs_adr_links_test.exs green: the README keeps its absolute links to the two numbered records it cited before.
• Sabotage of the basic usage block (one expected value changed) turned doctest README.md (1) red on the assertion; restored byte-equal and green.
• The introduction was swept against the manifest's avoided terms and the fixture-only example domains; no hit.
• No changelog fragment: changelog.d/README.md excludes documentation.

Direction check

This PR carries no record text: no ADR is added, edited or cited anew. The map's claims were checked against the pages they link at the head: each anchor (#running-a-program-from-elixir, #the-vocabulary-for-an-editor, #cross-language-siblings, #check-the-isa-version-before-you-run-a-stored-program) lands on the section its line names.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.